The invention relates to hybrid, including so called enhanced hybrid, step motors and has particular application to such motors having a 1.8 or 0.9 degree step angle, although it will be understood by those skilled in the art to have application to other step motors.
Step motors have been widely adopted for a variety of control systems. Such motors, are particularly attractive because they lend themselves to an open loop control structure and because they are readily operated with a microprocessor control. The open loop control structure is particularly advantageous since it eliminates the need for transducers attached to the motor to measure position, velocity or acceleration. The generally recognized step motor types include the variable reluctance step motor, the canstack motor and the hybrid step motor. Such motors have been known for at least 20 years. More recent developments include the axial airgap step motor and enhanced hybrid step motors. The axial airgap step motor designs have included velocity sensing coils integral with the motor. These coils provide damping for the step motor by velocity feedback. Such velocity coils produce sinusoidal voltages. Motors damped by such coils can respond with no significant overshoot and thus reduce the motion time. In addition, the damping also eliminates the possibility of resonances which can result in loss of synchronism. An example of the present state of the art is a paper entitled, "Damping A Two-Phased Step Motor With Velocity Coils" by Jacob Tal, Luciano Antognini, Pierre Gandel, and Norbert Veignat presented in the 1985 Incremental Motion Control Systems Society publication.
So called enhanced hybrid step motors have been manufactured which have rare earth permanent magnets inserted between the stator teeth. This produces a marked reduction in magnetic leakage. This design produces an increase in pullout torque of 50 to 60 percent over similarly sized conventional hybrid step motors. The present in vention has particular application to the hybrid step motor and the enhanced hybrid step motor.
The hybrid step motor has been widely used for the carriage drive and daisy wheel drive in printer mechanisms utilized in computer installations. Other applications for hybrid motors include those where the application requires that overshoot and ringout are minimized.
A limitation inherent in step motors is that with a typical number of rotor teeth, fifty rotor teeth for example, relatively moderate shaft speeds of even 1500 R.P.M. require that the motor be driven at an electrical frequency of over 1000 Hz. At these frequencies, the eddy current and hysteresis losses in the laminations become significant. At high speed, the losses in the magnetic material limit the performance of the motor.
When step motors having a large number of poles are used at output levels of over 2000 watts, the high inductance of such motors causes their power factor to become unacceptable; too much of the voltage capability of the drive circuit is spent on overcoming inductance, rather than the torque related E.M.F. A reduction in inductance by a factor of two, while maintaining all other parameters, would typically double the available high speed torque and the mechanical output power for a given Volt-Ampere rating of the driver circuit. Since in a typical step motor system, the driver electronics tends to be the most costly part, and a motor that offers better utilization of the available voltage and current is of substantial value. The price paid for simplicity of an open loop system may become too high.
A second problem in the known hybrid step motors is that high speed performance is limited by the losses in the magnetic materials of the rotor and stator. A reduction in these losses permits the effective use of the improved performance, due to lower inductance.
All step motors are vulnerable to a loss of synchronism. More particularly, the motor may over overshoot a stable position and then "lose" or "skip" one or more steps. Any loss of steps must be the equivalent of one or more full rotor tooth pitches or one or more electrical cycles. Such occurrences are unacceptable in an open loop control system.
It is an object of the invention to provide an improved hybrid step motor which has improved high speed torque and mechanical output power.
It is another object of the invention to provide an improved hybrid step motor which will not require any increase in the cost of driver electronics and which will have a higher output power even when driven with existing drive electronics.
It is another object of the present invention to produce a hybrid step motor which has lower inductance and thus less loss in the magnetic materials of the rotor and stator.
Still another object of the invention is to provide an apparatus which will have a physical construction which lends itself to the incorporation of velocity coils for detecting the E.M.F. of the motor to provide damping and, more generally, closed loop control.
It is still another object of the invention to provide a hybrid step motor construction which incorporates such velocity coils in a structure which is magnetically balanced and which produces a torque which is close to the torque which is produced in the same motor without the velocity coils.
Yet another object of the invention is to provide apparatus which is less vulnerable to loss of synchronism.